The present invention relates to vehicle detector monitors and methods. More particularly, the present invention relates to the field of traffic control systems in which vehicle detectors are used to determine the presence of vehicles at predetermined locations along a roadway or at an intersection or crossing within a specified control zone. Signals from the vehicle detectors are applied to a traffic controller to advise the controller of the presence of vehicles at those particular locations. This allows the traffic controller to permit all movements of traffic at an intersection or crossing to pass through that crossing in a safe and orderly fashion.
The use of traffic signal lights to control the flow of traffic, particularly at the intersection of two or more streets or highways, is well known. The traffic signal lights controlling traffic at an intersection are typically controlled by a local traffic controller which is programmed to produce command signals which are ultimately applied to the traffic signal lights. To control the various movements of traffic at an intersection, the traffic controller relies on "Call" signals sent to it from the vehicle detectors to determine the presence of vehicles at various key locations at the intersection. This information in the form of call signals from the vehicle detectors allows the controller to cycle the traffic signal lights at the intersection in a logical and orderly fashion. By allowing traffic movements only where needed, optimum traffic flow will be maintained.
The vast majority of vehicle detectors presently in use are referred to as "loop" detectors. These devices utilize a coil or loop of wire embedded under the road surface to provide the signal input to the detector. When a vehicle passes over the loop, the inductance of the loop is changed, and the vehicle detector senses this inductance change and provides a "call" signal output to the controller. A common problem with this type of detector is that the loop will either short circuit across itself or break open due to the abuse it is subject to while embedded under the road surface. For this reason, the loop or input device needs to be tested. Moreover, the vehicle detector itself is also subject to mechanical or electrical failure.
Typically, a vehicle detector and sensor combination will fail in one of two modes. Either the detector will fail in such a way as to cause its output to stay "on" continuously, which provides a continuous "call" signal to the controller whether there is a vehicle present at that detector location or not, or the detector will fail in such a way as to cause its output to stay "off" continuously, which will not place a "call" signal to the controller when a vehicle is present at that detector location.
The first mode of failure above-described may cause the traffic controller to continually cycle to that particular movement of traffic where the vehicle detector has failed, and may allow that movement its maximum allowable time even though there may be little or no traffic there. This ultimately takes away from time which would be allotted to other traffic movements and could cause traffic congestion.
The second mode of failure is potentially the most hazardous. After the vehicle detector fails to provide a "call" signal when a vehicle is present, the controller may never cycle to that movement of traffic to allow it to proceed through the intersection. In this situation, the driver of the vehicle may proceed through the intersection at his own discretion against a red signal output from the controller. This is potentially hazardous to the driver himself as well as other drivers at the intersection, and may be an unavoidable situation in the event of this type of failure.
It is, therefore, an object of the present invention to provide an improved fault detecting circuit and method for a vehicle detector system.
It is further an object of the present invention to provide an improved fault detecting circuit and method for a vehicle detector system which provides a monitoring function of a vehicle detector and sensor and gives visual indications of a fault condition.
It is still further an object of the present invention to provide an improved fault detecting circuit and method for a vehicle detector system which can detect shorted and open conditions in a roadway embedded vehicle sensor.
It is still further an object of the present invention to provide an improved fault detecting circuit and method for a vehicle detector system which can disconnect a defective vehicle detector and sensor combination from a traffic controller.
It is still further an object of the present invention to provide an improved fault detecting circuit and method for a vehicle detector system which can automatically supply a synthetic call signal to a traffic controller.
It is still further an object of the present invention to provide an improved fault detecting circuit and method for a vehicle detector system which provides for a self-test function for internal faults.